Fuel cells are devices that convert chemical reactants, namely an oxidant/oxidizer and a fuel, into electricity. Recent advances in fuel cell technology have increased their efficiency and lowered their cost, which has allowed fuel cells to compete with other more conventional types of energy conversion devices such as combustion engines and batteries. The variety in the types and sizes of fuel cells makes them versatile for use in a variety of applications, including providing electrical power for laptop computers, powering vehicles, and even providing electrical power for homes.
FIG. 1 illustrates a typical electrical power generator 100 that utilizes a fuel cell 102 to generate electricity. Electrical power generator 100 includes recycle system 112 for a fuel. Fuel cell 102 has a fuel inlet 104 for input of fuel into fuel cell 102 from a fuel source. Any fuel not consumed by fuel cell 102 for the generation of electricity exits fuel cell 102 through fuel outlet 106. Fuel recycle system 112 recirculates unconsumed fuel from fuel outlet 106 back to fuel inlet 104.
Fuel cell 102 also has an oxidant inlet 108 for input of oxidant into fuel cell 102 from an oxidant source. The oxidant will chemically react within the fuel cell with the fuel to produce electricity and a reactant product. The reactant product generated by the reaction of the oxidant and the fuel will exit fuel cell 102 through oxidant outlet 110. The reactant product may be discharged for disposal.
One type of fuel cell that has garnered a significant amount of research and interest is the polymer electrolyte membrane (PEM) fuel cell. The PEM fuel cell operates by supplying hydrogen to an anode, where a catalyst, typically a platinum-containing catalyst, separates the hydrogen into electrons and protons. The electrons that are separated from the hydrogen are transported to a cathode via an external circuit, which can be used to provide electrical power for a desired application/electrical load (electric motor, communication systems, propulsion systems, etc.). The protons are transported from the anode to the cathode through a solid electrolyte, namely a polymer electrolyte membrane (PEM). The PEM is a solid polymer and is typically made from a proton conductive fluoropolymer.
Oxygen is supplied to the cathode of the PEM fuel cell as pure oxygen or oxygen diluted with other gases. For example, the oxygen can be supplied to the cathode in the form of ambient air. At the cathode, the protons and electrons, aided by a catalyst, combine with the oxygen to produce water as a reactant product.
One problem encountered by some PEM fuel cells is a build up of inert gases (inerts) on the anode side of the fuel cell. The inerts diffuse into the fuel cell from the ambient environment because of the low partial pressure of nitrogen and other inerts present in the fuel cell system. Additionally, if the oxygen feed into the cathode contains inerts, for example if air is used, then the inerts in the oxygen feed will eventually diffuse from the cathode to the anode side of the fuel cell. The build up of inerts on the anode side of the fuel cell has the effect of displacing the hydrogen. The displacing of the hydrogen by the inerts keeps the hydrogen from contacting the catalyst and generating electrons and protons. When the inerts reach a high enough concentration, very little, if any, hydrogen contacts the catalyst and the fuel cell stops producing electricity.
Conventionally, the inerts problem is handled by periodic venting/purging of the anode side of the fuel cell. The venting/purging removes the inerts from the anode, thereby allowing the hydrogen access to the catalyst. However, when the anode side of the fuel cell is purged, valuable hydrogen is also purged. The amount of hydrogen purged with the inerts can be at least somewhat controlled to limit the amount of lost hydrogen by timing the purge cycle to efficiently remove the inerts. Even with efficient purging of the anode, however, a significant portion of the hydrogen is lost to purging of the inerts.
Depending on the application of the fuel cell, losing 10 percent of the hydrogen on every purge may be tolerable. For example, in automotive applications, losing 10 percent of the hydrogen on every purge is not a significant issue because the vehicles must be periodically re-fueled anyway. In closed loop systems such as space systems and the like, efficiencies are of major importance and losing 10 percent of the hydrogen each purge is not acceptable.
Thus, there is a need for an improved way of dealing with inerts in hydrogen fuel cells, particularly closed looped systems, that reduces or eliminates the purging of valuable hydrogen.